1. Field of the Invention
The present invention is related to an organic electroluminescent element. It is expected that an organic electroluminescent element is used for wide variety of applications. Applications of an organic electroluminescent element include a flat panel display used in a television, a PC monitor, portable units such as a portable telephone, a surface emitting light source, an illumination, and a light emission type advertisement body.
2. Description of the Related Art
Because an organic electroluminescent element has the following merit, an organic electroluminescent element is expected as a flat panel display as a substitute for a CRT and a liquid crystal display.
1. Viewing angle is wide.
2. Response speed is fast.
3. Power consumption is low.
An organic electroluminescent element is explained below.
The organic luminescent medium layer is sandwiched between two electrode layers (an anode layer and a cathode layer). A cathode layer and/or an anode layer are transparent. An electric current flows by applying voltage between both electrodes. Then light emission occurs in an organic light emission medium layer.
As for the organic electroluminescent element, quality deteriorates due to influence of atmospheric water vapor and oxygen. Therefore, an organic electroluminescent element is shielded from the atmosphere by covering an organic electroluminescent element by a metal can or a glass cap containing a desiccant.
In late years a top emission organic electroluminescent element is proposed to improve takeout efficiency of light emission of an active matrix type organic electroluminescent element. In substitution for a conventional sealing by a glass cap containing a desiccant, a barrier film having barrier property is provided to an organic electroluminescent element. An adhesive and a transparent sealing substrate are further put on an organic electroluminescent element. (Patent Reference 1).
In addition, in an organic electroluminescent element using a plastic film as a substrate, a barrier film of which barrier property is excellent is needed to shield an organic electroluminescent element from the atmosphere.
A barrier film of an organic electroluminescent element must have superior barrier property. Therefore, an inorganic film made of silicon dioxide, silicon nitride or the like is generally used as a barrier film. However, while a film is influenced by a substratum, a film layers on a substratum. Film defects such as a cavity and a pinhole of a film inside or a crack of a film occur due to a projection of an electrode and a substrate, and a particle. Then even if a film is thickened, and another film is laminated on the film formed beforehand, film defects cannot be suppressed. In addition, an organic material included in a cathode and an organic luminescent medium layer is very easy to react with water or oxygen. As a result of occurrence of the reaction, a non-luminescent point as referred to as a dark spot occurs. Therefore a life time of an organic electroluminescent element shortens.
Unevenness of a substrate can be coated by inserting an organic resin layer between inorganic laminated films. In addition, a defect position of an inorganic laminated film can be shifted by shielding a defect occurring in an inorganic film by an organic resin film (Patent Reference 2).
However, it is necessary to prepare a film formation chamber for an organic resin film. Cleaning of an organic resin film formation chamber is difficult. A particle is easy to occur. Therefore, there is a problem in mass production.
In addition, in this method, a defect remains in each inorganic film. Therefore, if the number of layers of laminated films is less than about 5 or 6, barrier property of an organic electroluminescent element is not sufficient.
Generally, a thicker glass (this glass is named counterbored glass) of which central part is processed to a recess form is bonded to an organic electroluminescent element with an adhesive such as epoxy resin so that the glass covers the cathode upper part. (Patent Reference 3)
Then an organic electroluminescent element is shielded from outside water vapor or oxygen by a counterbored glass.
However, in technique with the use of this counterbored glass, thin organic electroluminescent element cannot be made.
On the other hand, a display unit having the following characteristic is hoped for as an information display terminal: large size; a great number of pixels; fast response speed; and low power consumption.
It is said that it is necessary to use the following organic electroluminescent element in order to make such a display unit: an active matrix drive (thin film transistor (TFT) is used.); and top emission structure (light is taken out from an opposite side of TFT substrate.)
In making this organic electroluminescent element of top emission type, when sealing by a metal can and a counterbored glass is performed, light cannot be taken out from a top face.
Thus a method how an electrode is coated by a polymer film with a transparent barrier film is proposed. (Patent Reference 4)
However, it is necessary to form a thin film on a polymer film to raise barrier property of a polymer film itself when an electrode is coated by a polymer film. Therefore, the number of the processes increases. In addition, when a polymer film is stuck on an electrode, a bubble remains in a boundary surface. An organic electroluminescent element suffers damage by active gas inside a bubble.
Therefore, as a protection plate, the member which an adhesive is applied to a flat glass is used. And an organic electroluminescent element is sealed by sticking a protection plate on a cathode. (Patent Reference 5)
In this organic electroluminescent element, the surface of an organic electroluminescent element is covered by a rigid glass plate. Therefore, this organic electroluminescent element is superior in mechanical strength. In addition, manufacturing this organic electroluminescent element is easy. However, water vapor permeability of an adhesive itself is high. Therefore, this sealing method is inferior to a conventional sealing method using a cap. Moistureproofness of an organic electroluminescent element can be improved by mixing a desiccant with an adhesive (Patent Reference 6).
However, it has not resulted in fundamental solution.
In order to solve the above mentioned problem, a method how a protective film is formed on an organic electroluminescent element before applying an adhesive on an organic electroluminescent element is proposed. From a good balance between transparency and barrier property, inorganic material such as Al2O3, SiO2 and Si3N4 is usually used as a barrier film.
However, an organic electroluminescent element deteriorates when sealing ability of a barrier film is not sufficient. In other words this barrier film is a barrier film which is insufficient for a protective layer of an organic electroluminescent element.
Therefore, the conventionally proposed protective layer of an organic electroluminescent element with the use of a flat glass is insufficient for a seal of an organic electroluminescent element.
The present invention provides a barrier film superior in barrier property without a defect. In addition, the present invention provides an organic electroluminescent element without the occurrence and enlargement of a defect for a long term with the use of this barrier film.
In addition, according to the present invention, a barrier film which has hygroscopic property and water-vapor-proofing property can be easily formed. In addition, the present invention provides an excellent thin organic electroluminescent element without aged deterioration and the manufacturing method.
Patent Reference 1: Japanese Patent Laid-Open No. 2002-231443 Official Gazette
Patent Reference 2: Japanese Patent Laid-Open No. 2004-103442 Official Gazette
Patent Reference 3: Japanese Patent Laid-Open No. 05-109482 Official Gazette
Patent Reference 4: Japanese Patent Laid-Open No. 2004-79292 Official Gazette
Patent Reference 5: Japanese Patent Laid-Open No. 2002-216950 Official Gazette
Patent Reference 6: Japanese Patent Laid-Open No. 2003-303680 Official. Gazette